how to do PART III - Examining the Mechanism by which Voltage Gain is Produced B

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how to do PART III - Examining the Mechanism by which Voltage Gain is Produced

BIPOLAR JUNCTION TRANSISTOR COMMON EMITTER AMPLIFIE Description and Purpose: The common emitter amplifier is the most commonly used BJT single transistor amplifier especially when a voltage gain of greater than one is desired. It is especially useful in serving as an instructive exercise for the student to develop an understanding of the chain of events which cause input voltage to be amplified. +12v All amplifiers have limitations in terms of the dynamic range of the output. These in turn limit the range of appropriate input voltages. It is useful to be able to relate these limitations to the states of operation of the single transistor used to make this amplifier R1 100 K Co 10 uf 3.3 Ci V 10 u 01 CC m Pp zero offsel 1 Krtz R2 22 K Goals To learn to make voltage gain measurements on an amplifier To develop an understanding of how voltage gain is produced To practice the DC Troubleshooting procedure on an actual functional amplifier To observe the effects of clipping, and to relate these effects to the states of operation of the transistor 2N3904 Pin Out C B E

Explanation / Answer

Part III.

1. Here you have to observe the ac waveforms across the Base(vb) and Emitter(ve) of the BJT. Put the CRO channels in AC coupling mode so that the DC offsets will be removed from the signals. Now record the waveforms. Observation : ve is only few millivolts less than vb => ve follows vb.

2. Here you have to make a table of peak vb versus peak ve for different vin values. vb will be same as vin whereas ve will be slightly less than vb.

Important : while taking all these measurments CRO channels should be AC coupling mode, otherwise the ac waveforms will be clamped (with the DC biasing offset)

3. Now coming to the important point : How DC biasing desides the voltage gain

As you know, Voltage gain, Av = Vout/Vin for a CE amplifier Av -RC/RE.

To analyze this, we are going to change the DC biasing of the amplifier. Increase the voltage across base(VB) by increasing the value of R2 (You can keep a variable resistor -R2'' in series with the R2).

Now keeping the vin constant, vary the R2'' and observe how vout is changing (it increases till it reaches saturation)

Here you are changing R2'' , you are changing the DC biasing of the amplifier

R2'' can only be increased to a certain extent, after that the BJT goes out of active region and enters saturation.

This also can be observed (vout value drops )

4. Observe vc and vo with CRO in DC coupling mode (i.e, it shows AC+DC) and observe that the DC component in vc is blocked by the output capacitor Co and only AC component is present in vo.

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